(1) Field of the Invention
This invention generally relates to bi-directional communication systems and more specifically to communication systems capable of conducting bi-directional communications in an undersea environment.
(2) Description of the Prior Art
Acoustic communications in undersea applications are subject to multi-path effects in the water. Multi-path effects are produced by acoustic propagations from a transmission point that travel either directly to an underwater receiver or may reflect from the ocean surface and ocean floor or even areas of different temperature and density to create cancellation and distortion of the directly propagated transmission.
Some suggest that these multi-path effects can be overcome by the transmission of data over a number of different transmission frequencies. This improves the chances of clear communications as one or more of the transmitted signals may ultimately be received without severe multi-path distortion. However, such systems tend to be complex and difficult to implement. They also make certain assumptions about transmissions that may not be accurate in an actual operating environment.
There are a number of other approaches for undersea acoustic communications that vary with different applications. United States Letters Patent No. 4,563,758 (1986) to Paternostor discloses an underwater communicator device that permits acoustic communications between divers by using a voice synthesizer and an acoustic transducer. A display is provided to visually communicate a message. The diver can communicate stored messages by activating a single key or by keying in an actual message. Other preset messages can also be sent based on different sensors.
U.S. Pat. No. 5,018,114 to Mackelburg et al. (1991) discloses another type of acoustic communication system in which an operator has adjustable frequency diversity so data rates can be tailored to specific multi-path environments. Transmitted messages are sent with precursor transmission/reception synchronization data and transmission parameter data so the receiving communication end can recognize when message data starts by means of tone length as well as frequency diversity in the transmitted message. Timing is extracted from the data to compensate for Doppler shift.
U.S. Pat. No. 5,303,207 to Brady et al. (1994) discloses an acoustic local area network for oceanographic observation and data acquisition. A network node has telemetry equipment for transporting data to a final destination. Each of a plurality of sensors has an acoustic modem to transmit information to the network node. Transmissions are in the form of BPSK input signals. The data channels occupy a bandwidth of about 5-10 kHz while control channels occupy a frequency bandwidth of about 1 KHz.
U.S. Pat. No. 5,523,982 to Dale (1996) discloses communication apparatus for diver-to-diver communications. This system uses ultra-acoustic transmission means and reception means. When the transmission means is activated, a predetermined signal is transmitted that is suitable by reception at another diver's apparatus.
U.S. Pat. No. 5,469,403 to Young et al. (1995) discloses a digital sonar system that identifies multi-frequency underwater activating sonar signals received from a remote sonar transmitter. A transponder includes a transducer that receives acoustic waves, including the activating sonar signal, and generates an analog electrical receipt signal. This signal converts to a digital receipt signal that is cross-correlated with a digital transmission signal pattern corresponding to the activating sonar signal. A relative peak in the cross correlation value is indicative of the activating sonar having been received by the transponder. In response to identifying the activating sonar signal, the transponder transmits a responding multi-frequency sonar signal.
U.S. Pat. No. 6,058,071 to Woodall et al. (2000) discloses a magneto-inductive submarine communications systems and buoy to provide two-way signal communication between a submerged craft, such as a submarine, and a remote command station that may be airborne, on the surface or on land. A buoy released from the submarine and floating on the surface of the ocean and a satellite are included to complete bi-directional communications. Messages and commands between the submerged craft and the buoy are communicated by magneto-inductive messages signals and magneto-inductive command signals in an extremely low frequency to very low frequency ranges. Messaging command communications between the buoy and the satellite to the station are transferred via radio frequency signals or laser emissions.
Each of the foregoing techniques provides some method of undersea communication, but not a system that provides reliable undersea communications. United States Letters Patent Nos. 5,469,403 and 5,303,207 disclose bi-directional acoustic systems, however they do not use incoherent signal processing. What is needed is a system that provides reliable undersea communications at reasonable data rates